Actually, polish means remove a tiny bit of material
and make shiny. A felt wheel chucked into a cordless drill and some rubbing compound or some jeweler's rouge. You don't want to file away any material, just polish it, which will remove some material in the process.
The plug you asked about is a spring plug that usually comes on a 1911, but not on your's. You have a guide rod instead. If you had a standard 1911 with a guide rod (like a Springfield loaded), you could remove the guide rod and install the spring plug instead. I did that on my full size loaded model. To do that, you need a gun that has a barrel collet. Your Longslide has a bull barrel, which means it does not have a barrel collet.
The Longslide has a two piece guide rod for a very good reason. A one piece guide rod will not install from the backend, which is how you have to assemble the longslide. If the gun had a barrel collet, you would install the spring and guide rod (or spring and spring plug) last, by turning the collet and sliding the parts into the assembled gun. With a bull barrel, the barrel is bigger around and takes up the space in the slide that the collet would occupy, so an alternate method of installing the spring is used.
This alternate method just happens to be harder.
So did you get in on the thread where Triton is giving away 450SMC???? It's made for
your gun! It's still available if you missed the thread. they're giving away 20rd. boxes to the first fifty guys who email them through the thread they've got going here.